The quality of a structure, whether it is a house, apartment building, or commercial office building, is inextricably tied to its foundation. If the structure is not built on a proper foundation, the rest of the structure, even if properly constructed, is likely to show defects over time. When foundations are constructed directly on soils or on the ground, it often creates an unstable environment for the foundation. In addition, if these soils are active or expansive, the environment may be especially problematic. For example, in regions where the soil has a high percentage of active clay, expansion and contraction of the clay subjects the foundations to significant loads (e.g., forces) and potential movement.
Structures built on soils in certain regions may have had their slab foundations and walls displaced and damaged (e.g., cracked foundations and walls) as a result of differential expansion and/or contraction of the soil. Over time, engineers have developed systems and methods for designing foundations in an attempt to minimize damage due to soil movement. Some of these systems and methods include isolating heavy slab foundations from the active soils by suspending the slab above the ground using structural supports (e.g., helical piers, drilled shaft piers, pressed concrete or steel pilings, spread footings, natural rock, etc.) and lifting assemblies (e.g., lifting bolts, hydraulic jacks, air-inflatable jacks, electrical scissor jacks, etc.). The installation of supports and lifts to raise the slab foundation creates a protective void between the soil and the slab foundation. The void permits vertical expansion of the soil without subjecting the slab foundation to varying forces associated with the dynamic nature of soil. This method may also mitigate slab foundation failures due to seismic activity (e.g., earthquakes, tremors, etc.), which may cause the soil to move in a manner that can damage a slab foundation. U.S. Pat. No. 7,823,341 (the “'341 Patent”), HEIGHT-ADJUSTABLE, STRUCTURALLY SUSPENDED SLABS FOR A STRUCTURAL FOUNDATION, issued on Nov. 2, 2010, which is incorporated by reference herein, discloses a method of lifting a slab foundation using structural supports and lifting assemblies.
FIG. 1 shows a cross section of a prior art system for suspending slab foundations as described in the '341 Patent. As the '341 Patent describes, prior to forming slab foundation 170, structural supports 120 are installed into ground surface 110. Lifting assemblies 130 are installed on top of structural supports 120. Also, perimeter trench 140 is excavated along slab foundation 170's intended perimeter prior to pouring the concrete to facilitate the formation of perimeter beams 150 (e.g., an extension of slab foundation 170 into perimeter trench 140).
Typically, concrete is then poured within form boards that are placed along the perimeter of slab foundation 170. The poured concrete also flows into perimeter trench 140, which defines the depth and thickness of perimeter beams 150. Large excavators are typically required to create perimeter trench 140 to ensure that the resulting perimeter beams 150 are of sufficient depth and width to mitigate concrete's low shear strength and inherent structural weaknesses related to creep and/or shrinkage during the curing process. In most situations, foundation reinforcements are installed, prior to pouring the concrete, within the area where the concrete will be poured (e.g., the foundation area and the trenches for the perimeter beams). Installing foundation reinforcements (e.g., post-tension cables, steel, glass, plastic fibers, or hand-tied rebar) may introduce additional labor (e.g., workers cutting and installing the reinforcements) and material costs. Once the concrete is poured to cast slab foundation 170 and perimeter beams 150, the concrete is allowed to cure, and as the concrete cures and strengthens, it is secured to lifting assemblies 130 atop structural supports 120. Although commonly implemented along the periphery of a slab foundation 170, the techniques for forming perimeter beams 150 can also be implemented within the interior of slab foundation 170. For example, perimeter beam 150 may circumscribe a leave out portion of slab foundation 170 designed for an internal garden.
After slab foundation 170 is formed on ground surface 110, lifting assemblies 130 are used to lift slab foundation 170 to a desired height above ground surface 110, thereby creating void 180 between ground surface 110 and slab foundation 170. To lift slab foundation 170, force (e.g., torsion, expansion, or other forces related to the type of lifting mechanisms used) is applied to lifting assemblies 130. On application of sufficient forces, lifting assemblies 130 will raise slab foundation 170 up from ground surface 110. As lifting assemblies 130 lift slab foundation 170, void 180 is created. After the lifting of slab foundation 170, ground surface 110 may expand within void 180. Thus, with void 180 in place, expansion of ground surface 110 within interior space 142 does not result in varying forces on slab foundation 170 resulting from the dynamic nature of soil. However, void 180 is vulnerable to lateral soil migration from beyond the periphery of slab foundation 170 due to soil movement, soil liquefaction, etc. Accordingly, perimeter beams 150, which are formed as an extension of slab foundation 170 and are raised along with slab foundation 170, circumscribe void 180 and may operate as a barrier to lateral migration and/or ingress of soil into void 180 (e.g., lateral migration of soil from beyond the periphery of the slab structure). However, the process of creating perimeter beams 150 is labor intensive and adds significantly to construction time and material costs.
Sometimes, soil retainers may be used to in combination with perimeter beams to mitigate soil migration, but conventional soil retainers can be cumbersome. Often, conventional soil retainers may be installed within perimeter trench 140 such that pouring concrete into perimeter trench 140 results in perimeter beams 150 forming against the soil retainers. The retainers are typically used to prevent soil migration during the formation of slab foundation 170 and perimeter beams 150. Once slab foundation 170 and perimeter beams 150 are raised, the retainers may be raised along with perimeter beams 150 and merely supplement the function of perimeter beams 150. Conventional retainers may be also installed by excavating soil around perimeter beams 150, installing the retainers against perimeter beam 150, and re-compacting soil around perimeter beam 150. However, these conventional soil retainers take for granted, and thus do not address, the material and labor costs and structurally weakness related to perimeter beams 150.